The invention relates to oil burners of the type comprising an oil atomizer device; a plate arranged downstream of the oil atomiser device and having at least one opening therein providing a throughway for the atomized oil and also being the sole throughway for combustion air; a mixing tube arranged co-axially at a distance downstream of said plate and having a diameter greater than the diameter of a circle bounding the throughway in said plate, and a flame tube co-axially surrounding said mixing tube and having a length such that the flow of combustion gases downstream of said mixing tube is applied against the inner wall of said flame tube. The invention also relates to gaseous fuel burners of the type comprising an air supply chamber and a gaseous fuel supply means having outlet openings through which the gaseous fuel is introduced into the stream of combustion air. The burners are for installation in heating and steam producing boilers.